Syringes of the subject type typically include a barrel for containment of a liquid medicament or solution, a plunger tip slidably disposed within the syringe barrel, and a plunger rod secured to the plunger tip. During automated assembly, a cap is secured to an end of the barrel, the barrel is filled with liquid solution, and the plunger tip is placed in the barrel over the solution. The plunger rod is then secured to the plunger tip. To use the syringe, the cap is removed from the end of the barrel and a needle is threaded onto the barrel. The plunger rod and plunger tip are then manually pushed into the barrel to eject liquid solution from the end of the needle. It is a general object of the present invention to provide a method and apparatus for injection molding the plunger rod that employ a simple mold structure and simple opening/closing actions at the mold structure. Another object of the present invention is to provide a method and apparatus for injection molding a syringe plunger rod that achieve improved plastic material flow and improved filling of the threaded end portion of the plunger rod that is secured to the plunger tip. Another object of the present invention is to provide a plunger rod for a syringe manufactured employing such method and/or apparatus.
A plunger rod for a syringe in accordance with a presently preferred embodiment of the invention is of integrally injection molded plastic construction having an elongated body with a coaxial end portion. An interrupted or segmented external thread extends around the end portion for threaded receipt into a plunger tip that is slidably received in a syringe barrel. A flat extends chordally across the end portion of the rod interrupting the external thread. The plastic, of which the plunger rod is molded, is gated into the mold for the plunger rod at the flat that extends across the rod end portion, leaving a gate mark or vestige on the flat. Gating of the plastic material into the portion of the mold that forms the threaded end portion of the plunger rod achieves improved flow and improved filling of the external thread, while employing a simple and economical mold construction. The gate vestige does not interfere with threading of the plunger rod end portion into the plunger tip. In the preferred embodiment, the end portion of the plunger rod is cored during molding to provide a C-shaped cross section at the rod end portion, which improves dimensional stability during material shrinkage after molding.
A method of making a plunger rod for a syringe, in accordance with a second aspect of the present invention, contemplates providing a mold having at least one mold cavity for molding a plunger rod having a body, an end portion with an external thread and a flat extending chordally across the end portion interrupting the external thread. Plastic material, such as polypropylene, is gated into the mold cavity at this flat such that the plastic material flows to fill the cavity, and the plunger rod is then removed from the mold cavity. A mold for injection molding a plastic syringe plunger rod in accordance with a third aspect of the invention includes a first mold section having a body and a gate insert, and a second mold section having a body and a core insert. The first and second mold sections together form at least one mold cavity. The gate and core inserts form the portion of the mold cavity within which the end portion of the plunger rod is molded. This portion of the mold cavity has a C-shaped axial cross section, with the gate insert forming a flat in a central portion of the C-shaped cross section. Plastic material is gated through the gate insert radially into the end portion of the mold cavity, and then flows axially into the cavity to mold the plunger rod.